Coccidiosis is a disease caused by infection with one or more of the many species of coccidia, a subdivision of the phylum Protozoa. The coccidia are intracellular parasites which can infect a wide range of hosts and may result in severe economic loss to the sheep, goat, cattle, swine and poultry industry. Indeed, coccidiosis resulting from infection with Eimeria species has caused economically devastating losses to the poultry industry. Among domesticated birds, chickens are the most susceptible to the economic losses from coccidiosis, although losses can also occur with turkeys, geese, ducks, and guinea fowl. Coccidiosis also produces serious losses in pheasants and quail raised in captivity. Coccidiosis may be acute and characterized by devastating flock mortality or the disease may be chronic and characterized by a lack of weight gain.
Poultry are infected by coccidia following ingestion of the vegetative stage of the parasite, the sporulated oocyst. The infective stage, the sporozoite, is released in the intestine where it rapidly invades epithelial cells subsequently undergoing several generations of rapid intracellular asexual multiplication before entering the stage of sexual differentiation leading to the production of oocysts which are shed in the droppings. Low level infection with any of the Eimeria species (spp.) results in a protective immunity to reinfection. This has suggested that coccidiosis may be controlled by the use of live vaccines. The development of effective vaccines has been slowed by the complex life cycle of Eimeria and the fact that post infection immunity is generally species specific (Long and Rose, Worlds Poultry Sci. J. 38: 85-96, 1982). It has been shown that the infection must progress as far as the development of asexual stages before immunity is induced. Thus, the immunizing antigens for at least some species are likely to be contained in the asexual stages (Rose and Hesketh, Parasitology 73: 25-37, 1976). However, the sporozoite stage appears to have little immunizing value (Rose, M. E., 1982, Biology of the Coccidia, P. L. Long, ed., University Park Press, Baltimore, p. 329, 1982).
Previous attempts to immunize chickens with non-viable Eimeria components have been unsuccessful. Parenterally administered soluble Eimeria antigens failed to reduce subsequent challenge with infective sporozoites (Long and Rose, Exp. Parasitology 16: 1-7, 1965). Conversely, solubilized E. tenella sporozoite antigens have been used to protect chickens against challenge with E. tenella, Schenkel et al., European Patent Application No. 0135712, while solubilized merozoite antigens have been regarded as a potential vaccine, Schenkel et al., European Patent Application No. 0135073.